THE GUT-BRAIN AXIS
The human body is a complex ecosystem where various systems interconnect and influence each other. In recent years, scientific research has shed light on the intriguing relationship between the gut and the brain. The gut-brain axis refers to the bidirectional communication network between the gastrointestinal tract and the central nervous system. This intricate link allows the gut to communicate with the brain and vice versa, influencing various aspects of mental and emotional well-being.
The gut houses trillions of microorganisms, collectively known as the gut microbiota, which play a crucial role in maintaining the balance of this axis.
Beyond their impact on gut health, Probiotics have also shown promising effects on brain function. Research suggests that some specific strains of probiotics, only few of them, can modulate the gut-brain axis through various mechanisms.
Neurotransmitter modulation: Probiotics has been shown to produce neurotransmitters such as gamma-aminobutyric acid (GABA). GABA is an inhibitory neurotransmitter that helps regulate neuronal activity in the brain. By increasing GABA production, probiotics may have a calming and anxiety-reducing effect on the brain. Short-chain fatty acid (SCFA) production: Probiotics can enhance the production of SCFAs in the gut, such as butyrate. SCFAs are byproducts of bacterial fermentation of dietary fibers. Butyrate, in particular, has been associated with improved cognitive function and mental health. It can cross the blood-brain barrier and provide energy to brain cells, as well as exert anti-inflammatory effects that benefit brain health. Modulation of the gut microbiota: Probiotics can influence the composition and diversity of the gut microbiota. A balanced and diverse gut microbiota is crucial for optimal brain function. By promoting a healthy gut microbial ecosystem, probiotics indirectly supports brain health. Immune modulation: Probiotics can modulate the immune response in the gut. Chronic inflammation in the gut can lead to neuroinflammation and contribute to the development of brain disorders. By reducing gut inflammation, probiotics may help mitigate inflammation-related brain conditions. Metabolite production: Probiotics can produce various metabolites, including vitamins and enzymes. These metabolites may have indirect effects on brain function by supporting overall health and providing essential nutrients required for brain metabolism.
In a study conducted by Hu et al. (2018), dietary supplementation of a probiotic reduced aggressive behaviors in laying hens. Additionally, in mice, supplementation with probiotics, and their action on the microbiome, led to a release of neuroendocrine factors that subsequently affected the relative abundance of serotonin (the hormone of well-being) and tryptophan. Tryptophan, an essential amino acid, is a precursor in the bio-synthesis for serotonin and melatonin. Serotonin (5-hydroxytryptamine) is best known as a neuro-transmitter critical for the development and proper function of the central nervous system. Remarkably, 90% of the body’s serotonin is produced in the intestine by enterochromaffin cells (in purple in the picture below). Enterochromaffin (EC) cells (also known as Kulchitsky cells) are a type of enteroendocrine cell, and neuroendocrine cell. They reside alongside the epithelium lining the lumen of the digestive tract and play a crucial role in gastrointestinal regulation, particularly intestinal motility, and secretion.
Serotonin can travel through the bloodstream or via nerve fibers to reach different regions of the brain. It is involved in regulating mood, appetite, sleep, and other cognitive functions. The serotonin released by EC cells can affect specific receptors in the brain, influencing neurotransmission and neuronal activity.
Enterochromaffin (EC) cells are also reactive to mechanosensation, which is the case in the peristaltic reflex of the gut and can be stimulated by a bolus moving through the bowel. Upon activation, EC cells release serotonin to act upon serotonin receptors on ENS neurons. Dependent on concentration, serotonin can then modulate peristaltic contraction and secretion through activation of smooth muscle and glands, respectively.
It's worth noting that while serotonin is mainly produced in the gut and the brain, it is a systemic neurotransmitter that affects multiple organs and systems throughout the body, including the cardiovascular, immune, and gastrointestinal systems.
Several publications report a strong positive correlation between health of the intestinal mucosa and production of serotonin; the healthier the intestinal mucosa, the higher the concentration of serotonin produced by the birds.
In some recent researches, Christian Hansen, a company specialized in developing probiotics, demonstrated an increase of production of serotonin in broilers at 40 days of age when fed with probiotics in their diets.
Serotonin can travel through the bloodstream or via nerve fibers to reach different regions of the brain. It is involved in regulating mood, appetite, sleep, and other cognitive functions. The serotonin released by EC cells can affect specific receptors in the brain, influencing neurotransmission and neuronal activity.
Enterochromaffin (EC) cells are also reactive to mechanosensation, which is the case in the peristaltic reflex of the gut and can be stimulated by a bolus moving through the bowel. Upon activation, EC cells release serotonin to act upon serotonin receptors on ENS neurons. Dependent on concentration, serotonin can then modulate peristaltic contraction and secretion through activation of smooth muscle and glands, respectively.
It's worth noting that while serotonin is mainly produced in the gut and the brain, it is a systemic neurotransmitter that affects multiple organs and systems throughout the body, including the cardiovascular, immune, and gastrointestinal systems.
Several publications report a strong positive correlation between health of the intestinal mucosa and production of serotonin; the healthier the intestinal mucosa, the higher the concentration of serotonin produced by the birds.
In some recent researches, Christian Hansen, a company specialized in developing probiotics, demonstrated an increase of production of serotonin in broilers at 40 days of age when fed with probiotics in their diets.
To illustrate the correlation between probiotics and brain health, we monitored broilers when fed with probiotics. We observed that the group fed with probiotics tend to stand longer than the negative control group and are significantly less skittish (easier to touch).
Probiotics is a generic word including millions of different strains. As example, it is estimated that the bacillus subtilis species contains several hundreds of different strains. These strains can vary in their genetic makeup, physiological characteristics, and functional properties. They may differ in their ability to produce enzymes, metabolites, and other bioactive compounds. So we should be very careful when selecting a probiotic. We need to ask probiotics manufacturers some guarantees regarding the strains that they are marketing.
It's worth noting that the classification and characterization of Bacillus subtilis strains continue to evolve as scientific research advances. New strains are continually being discovered and studied, expanding our understanding of their diversity and potential applications.